This invention relates to compositions of matter suitable for laser use and to laser systems utilizing such compositions. More specifically, the invention relates generally to laser materials of the coupled ion type and particularly to devices generating coherent radiation in the wavelength region greater than 1.4 microns, the eye safe region.
In the prior art, lasers which were able to operate at frequencies in the eye safe region suffered from either low efficiency, high threshold starting, inability to operate at room temperature, and the requirement for cryogenic cooling; or from combinations of these problems. This is because lasers operate at fixed frequencies which are determined by the energy level scheme of the materials from which the lasing element is fabricated.
Possible eye damage from inadvertant exposure to coherent radiation produced by lasers operating in the region between 0.2 and 1.4 microns, in which region energy is transmitted by the eye and focused on the retina causing injury and possible blindness is an important consideration when using lasers in nonlaboratory environments where safety cannot be readily controlled; such as in range finders, illuminators and designators and in commercial applications such as surveying construction alignment, and communications. Lasers of the prior art, which do not suffer from the above-mentioned deficiencies, operate in the non-eyesafe region below 1.4 microns.
The most common optically pumped solid state lasers of the prior art are ruby and trivalent neodymium (Nd.sup..sup.+3) as a dopant in a host material of yttrium aluminum garnet (YAG) (Y.sub.3 Al.sub.5 O.sub.12) or glass. The operating frequency of the ruby laser is approximately 0.694 microns and that of the YAG laser is approximately 1.06 microns, hence their use constitutes an eye hazard.
An approach of the prior art to obtain eye safe laser operation has been the use of other lasing ions having laser transitions at longer wavelengths than either ruby or YAG, the most notable examples being trivalent erbium (Er.sup..sup.+3) as a dopant in glass, which has an operating frequency of 1.54 microns, trivalent erbium as a dopant in YAG which operates at 1.65 microns and trivalent holmium (Ho.sup..sup.+3) as a dopant in YAG which operates at 2.1 microns.
Another prior art technique to obtain laser operation in the eye safe region has been to down-convert the emission of a laser, for example, the 1.06 micron Nd.sup..sup.+3 emission, to longer wavelengths using either parametric effects or a Raman active medium.
These techniques have serious shortcomings in that while operation at eyesafe frequencies is achieved, the operation is at low efficiency. For example, both Er.sup..sup.+3 and Ho.sup..sup.+3 lasers have relatively high thresholds of oscillation at room temperatures, and must be cryogenically cooled to achieve low threshold oscillation, the threshold of oscillation being the energy required to achieve a population inversion. If the threshold for laser action is high the laser must be pumped at higher energies, hence the overall efficiency of these lasers is low at room temperature. Since efficient operation at room temperature is desirable, there have been attempts to improve the optical pumping efficiency thereby reducing the threshold of the Er.sup..sup.+3 and Ho.sup..sup.+3 lasers by the addition of various codopant ions; however, there are few satisfactory codopants for Er.sup..sup.+3. Other paramagnetic ions have been unsuitable for efficient, high-power laser operation in the eye safe region.